1. Field of the Invention
The present invention relates to a color signal demodulation circuit and, more particularly, to a color signal demodulation circuit which can demodulate a modulated color signal corresponding to the different chrominance subcarrier frequencies. A corresponding demodulation method is also disclosed.
Korean Patent Application No. 93-2762 is incorporated herein by reference for all purposes.
2. Discussion of Related Art
In the conventional color signal demodulation circuit, an input color signal (.omega..sub.sc +.omega..sub.m) are multiplied with the carrier signal for use in demodulation having the same frequency (.omega..sub.sc) with the subcarrier of an input color signal. Then, the unnecessary second harmonic signal is removed by a low-pass filter, to thereby demodulate the color difference signal R-Y and B-Y or baseband frequencies. This process can be explained in more detail with reference to FIG. 1.
FIG. 1 is a block diagram of the conventional color signal demodulation circuit, wherein only the color burst signal is output via a burst gate 10 among all of the input color signals after being separated into luminance and chrominance signals and sampled.
The color burst signal output from burst gate 10 is applied to a phase difference detector 20, where its phase is compared with that of a carrier signal generated and fed back from a downstream carrier generator 30. The phase difference resulting from the comparison is output as a voltage signal to carrier generator 30, which oscillates at 3.58 MHz, i.e., the color subcarrier frequency, in the case of an exemplary NTSC system. As a result, first and second carrier signals (sin.omega..sub.sc t and cos.omega..sub.sc t) for use in the demodulation are output.
In a color difference signal demodulator 40, the first and second carrier signals which are for use in demodulation which are output from carrier generator 30, are input. The fw1 color signal input via a modulated color signal input terminal is demodulated into color difference signals. In a first multiplier 41, the first carrier signal for use in the demodulation output from carrier generator 30 and the input modulated color signal (C) are multiplied, and the output signal of first multiplier 41 is output as the R-Y signal after filtering in first low-pass filter 43.
In a second multiplier 42, the input modulated color signal (C) and the second carrier signal for use in the demodulation are multiplied, and the output signal of second multiplier 42 is output as the B-Y signal by a serially connected second low-pass filter 44.
When various kinds of input color signals, whose carrier frequency and bandwidth are different, are to be processed, low-pass filters, which respectively correspond to the subcarriers of the color signals to be demodulated, are needed for use as a rear stage of the demodulation circuit.
For example, the frequency characteristic of an input signal X is shown in FIG. 2A, assuming that the signal X has a center frequency .omega..sub.sc of 1 MHz and a bandwidth .omega..sub.m of 2 MHz. Furthermore, the frequency characteristic of an input signal Y is shown in FIG. 2B, assuming that the signal Y has a center frequency .omega..sub.sc of 2 MHz and a bandwidth .omega..sub.m of 4 MHz.
When a 1 MHz carrier signal for use in demodulation is multiplied with the X-signal, the frequency characteristic shown in FIG. 2C is generated. Meanwhile, a 2 MHz carrier signal for use in the demodulation is multiplied with the Y-signal, and the frequency characteristic shown in FIG. 2D is generated.
Accordingly, a low-pass filter whose cut-off frequency is 1 MHz is used to produce the final X-signal, and a low-pass filter whose cut-off frequency is 2 MHz is used to produce the final Y-signal, thereby demodulating an original signal.
It will be noted that, when the frequencies of carriers of input color signals are different, other low-pass filters having different cut-off frequencies, respectively, are absolutely necessary. Therefore, component sharing between circuits is impossible. The number of circuits has to be enlarged in order to support a varied manufacturing facility. Furthermore, interchangeability with other systems is poor.
An FM demodulation device is disclosed in EP 399758, where FM demodulation device delays the phase of an FM-modulated signal using a so-called Hilbert transformer, to thereby remove the undesirable high frequency component which is higher than the carrier signal by a predetermined multiple using a low-pass filter, while improving the S/N ratio. Since the low-pass filter, which removes the high frequency component higher than the carrier signal by two times, is used in demodulating the modulated signal in the above-mentioned device, this device also dictates the use of low-pass filters having varying cut-off frequencies depending on the frequency of the carrier signal.